#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>

int main(){
	float N, i;
	double sum = 0.0;
	scanf("%f", &N);
	int f = 1;
	for (i = 1; i <= N; i++){
		sum = sum + f*i / (2 * i - 1);
		f = -f;
	}
	printf("%.3lf", sum);
	return 0;
}